1. Field of the Invention
This invention relates to exterior building wall systems utilizing multiple wall panels forming horizontal and vertical wall joints. Each individual wall panel consists of an essentially flat exterior facing member made of either coated metal plate or finished stone and four perimeter members which are structurally connected to the facing member. The wall joint design of this invention achieves better airtight and watertight performances.
2. Description of the Prior Art
Metal plate and stone wall systems have been used in the exterior wall construction for many years. It is known that there is a substantial air pressure differential between the exterior and the interior of most of the modern buildings. It is a common industrial practice to specify the watertight performance test of a wall system under a differential pressure of 20% of the positive design wind load with a minimum of 6.24 psf simulating a 50 mph wind velocity. In the prior art wall joint designs, the wall joint seals are located on or near the exterior wall surface. These sealant locations are subjected to the exterior running water, therefore, water will infiltrate through the sealant if pin holes exist in the sealant due to the effect of the differential pressure. Therefore, to prevent water from infiltrating through the wall joint, the sealant quality must be perfect (i.e. 100% airtight). However, perfect seal is difficult if not impossible to achieve and to endure due to the variation of workmanship and the relative joint movements produced by wind and thermal loads. Recognizing the difficulty of achieving perfect seal condition, all the successful prior art designs utilized an internal gutter system with drainage holes to collect and to drain the water infiltrated through the wall joints. The design principle of the internal gutter system is to control and to drain the water leaked through the wall joints. The system design is to allow water leakage but to put the leakage under control such that no interior water damage will be caused by the leakage. This condition is known as controlled leakage. The controlled leakage condition is acceptable in the watertight performance test. The drawbacks of the prior art internal gutter system are itemized below.
(1) The drainage holes are the linkages between the interior air and the exterior air and thus they are the source of air leakage which will reduce the thermal efficiency of the building.
(2) If the drainage holes are subjected to the exterior running water, the water will be sucked inwardly through the drainage holes due to the differential pressure. In this case, the drainage holes are the source of water leakage prior to the intended design function of drainage.
(3) Since the drainage holes are linking between the exterior air and the interior air, the water head inside the internal gutter must be built-up to overcome the differential pressure before outward drainage can take place. This gutter water head necessitates the following three design considerations. First, the gutter leg height must be larger than the expected water head to prevent overflow. Second, the butt joint of the internal gutter are more vulnerable to uncontrolled leakage due to the water head effect. Third, sustained differential pressure means that the water inside the internal gutter can only be dried out by evaporation resulting in maintaining a high relative humidity in the wall cavity. Therefore, a vapor barrier is normally used to protect the insulation installed behind the internal gutter system adding to the cost.
(4) Since the internal gutter is open on the interior side, the drainage holes are vulnerable to clogging due to the deposit of foreign materials during the interior construction. For example, it is often that the interior fireproof spraying is executed after the enclosure of the exterior wall. In this case, it is often to see that the internal gutter system is clogged by the overspray of the fire proofing material.
(5) Due to the effect of the differential pressure, the size of the drainage hole must be substantial for effective drainage. The larger the drainage hole, the better the drainage function. However, the smaller the drainage hole, the better the thermal efficiency. Apparently, the internal gutter system has created the above two contradicting design objectives.
(6) When the exterior air is being sucked through the drainage holes and the water inside the internal gutter, it creates the same effect of boiling water in which some water drops may jump out of the internal gutter system and become uncontrollable. To lessen this effect, the prior art designs utilized either a baffle block or a shielding plate at the location of every drainage hole.